When I first saw the sepia-toned posters heralding a new biopic of British author J.R.R. Tolkien, I feared the whole enterprise would be a bit too – precious. What a relief, then, to find the film almost entirely concerned with what he was like in his 20s, long before writing The Hobbit, and decades before The Lord of the Rings.

And Oh! What a rogue and peasant slave was he – well, at least by the standards of Edwardian England. Knocking about at King Edward’s School in Birmingham and later at Oxford University, Tolkien and his mates would drink tea at all hours, lip-synch to Wagnerian opera and in one instance bring open bottles of wine aboard an out-of-service omnibus. Scandalous!

He was also infatuated with Edith Bratt, a fellow orphan at the boarding house where Tolkien and his brother moved after the death of their parents. “She’s not even Catholic,” grouses his priest and guardian (Colm Meaney), but that’s no impediment to true love. If your image of Tolkien is the late-in-life portrait with a pipe, here’s your chance to recast him from fusty to lusty.

Nicholas Hoult takes on the role of John Ronald Reuel, with Lily Collins as Edith. Though somewhat confusingly, they and Tolkien’s schoolmates are at first portrayed by younger actors. It’s odd, even disconcerting – just as we’re getting to know the fellowship of the TCBS (Tea Club and Barrovian Society), they get replaced by a new bunch who look too young to be playing 30 and too old to be 20. (Hoult is 29.)

But that is one peril of the biopic. Another is that the family and estate of your subject will release a statement “to make clear that they did not approve of, authorize or participate in the making of this film and do not endorse it or its content in any way.”

But sometimes you have to soldier on regardless. As Bilbo Baggins once said to his nephew Frodo: “Don’t adventures ever have an end? I suppose not. Someone else always has to carry on the story.”

This one is being carried on by Finnish director Dome Karukoski in his English-language debut, and cuts between Tolkien’s tour of duty in the Great War, when so many of his generation were lost, and his young adulthood leading up to the conflict, which clearly left its imprint on the literary battles of Middle-earth. In one scene, you’re certain he’s suffering from some sort of madness, imagining fire-breathing dragons in the trenches, before a change in perspective reveals them to be all-too-real Germans wielding flamethrowers.

But if he was scarred by battle, he was also marked by a lifelong love of language that is tricky to show on film. But the movie manages, beginning with the scene in which Tolkien’s classmates snatch his copy of a Chaucer book from which they are reading aloud. When his turn comes, you expect him to cry thief, but instead he recites the Middle English from memory, and with perfect pronunciation.

Another achingly sweet scene finds the young Tolkien holding forth on the beauty of the phrase “cellar door” with Edith, who gamely argues that words owe more to their connotations than their phonaesthetics. This is also when you realize that theirs is a match made in philological heaven.

Tolkien is not a perfect biopic by any means – the end rattles on for almost as long as Peter Jackson’s Rings finale – but moments like this will carry you over any narrative rough patches. Fans of the author should find fellowship in their enjoyment of it.